Mould loves the damp
If you've stored your books improperly, you may have set yourself up for an environmental issue with mould.
You can often find evidence of mould if you know what you're looking for. There are thousands of different kinds of moulds, and some indeed will grow on paper-look for the signs of dusty spores, fungal elements, even the 'smell'. Bring a sample to a librarian or try a UV lamp if ur not sure.